In a colored cathode ray picture tube a shadow mask or aperture mask is located between the electron guns at the rear of the tube and the phosphor coated face plate at the front of the tube. Electron beams pass through tiny openings or apertures in the shadow mask and impinge upon suitable color producing phosphor dots, a triad one dot for each of the three primary colors. During operation of the picture tube the shadow mask openings are used as a guide for the electron beams.
In order to manufacture the shadow masks for use in the cathode ray tubes that are used in color monitors it is necessary to proceed through a number of steps to etch a metal web into a shadow mask having the small precision openings of proper dimensions and proper location. In a typical shadow mask manufacturing process one has to first make a photographic printing plate that is referred to as a photo printing plate for use in forming an etchant resist pattern in a layer of etchant resist located on the surface of a metal web. One of the methods used to create a photo printing plate involves applying a layer of a light sensitive emulsion to a sheet of flat glass. The emulsion layer is exposed to a light source to form a first etchant resist pattern in the emulsion. The emulsion is then developed to produce a master etchant resist pattern for defining the etchant resist pattern in the layer of etchant resist located on a metal web or sheet. Since precision etching of small openings usually requires selective etching from both sides of the metal web a second photo printing plate is created for laying out a second etchant resist pattern in the layer of etchant resist that is located on the opposite side of the metal web. Once the photo printing plates are made the user positions the photo printing plates in register on opposite sides of the metal web and in contact with the respective layer of etchant resist on the metal web. Next, one draws the photo printing plates into intimate contact with the etchant resist by evacuating the air between the photo printing plates and the layers of etchant resist on the metal web. Next one selectively exposes the light sensitive etchant resist layer through the photo printing plates. After exposing the light sensitive etchant resist to a suitable light source one develops the etchant resist. During developing, the soluble portions of the etchant resist are removed leaving a pattern of openings in the etchant resist. The openings in the etchant resist permit an operator to use an etchant to etch the metal web in the areas not protected by the etchant resist.
A commonly used light sensitive etchant resist is fish glue which is applied as a liquid and allowed to dry forming a layer of etchant resist that has a thickness of about 3 to 10 microns.
One of the problems in accurately etching openings in a metal web is the difficulty in accurately laying out the etchant resist pattern in the fish glue layers on the metal web. Two types of photo printing plates are used in the art. In the more accurate but costly process a very thin etchant resist master pattern is formed by deposition of a thin (less than 0.5 microns) layer of a metal oxide on a glass plate. In the less costly but less precise etchant resist pattern a master etchant resist pattern is formed in an emulsion layer about 3 to 5 microns thick. The actual opaque pattern formed with emulsions sits slightly embedded in the surface of the emulsion and projects outward from the emulsion surface a distance of about one micron. Since the accuracy of the image formed in the etchant resist layer on the metal web is a function of how close the pattern on the photo printing plate is to the etchant resist layer it is extremely important that the etchant resist master pattern on the photo printing plate is in as close proximity as possible to the layer of etchant resist.
In the emulsion type printing plate one coats a glass plate with a layer of light sensitive emulsion that is about 3 microns thick. Next a primary master pattern having opaque areas and nonopaque areas is placed over the layer of emulsion. Then the light sensitive emulsion is exposed to light and developed to form an etchant resist pattern of opaque regions on top of the clear or non opaque emulsion layer. After developing the emulsion on the photo printing plate one has a photo printing plate with a etchant resist master pattern formed by a relatively soft clear compressible emulsion base with an opaque master pattern projecting about 1 micron above the top surface of the clear emulsion. To expose and develop the light sensitive etchant resist on the metal web one draws the photo printing plate with the emulsion layer and master pattern into intimate contact with the etchant resist through application of a vacuum to the region between the emulsion layer and the etchant resist. The projections from the surface of the emulsion form channels that permit the air to be evacuated from the central regions of the photo printing plate.
One of the problems with the use of thick soft emulsions for the master pattern on a photo printing plates is that the opaque master pattern projections on the emulsion form a gasket like seal with the etchant resist on the metal web. If there are any enclosed regions defined in the photo printing plate the gasket like seal makes it difficult to evacuate the air from between the emulsion layer and the etchant resist. This problem is particularly acute along the region of the master pattern that is referred to as the strip line. The strip line is a an opaque band encircling a dot array pattern located on interior portion of the master pattern. During the exposing and developing of the etchant resist pattern on the etchant resist the opaque master pattern projection of the strip line is drawn into contact with the etchant resist by evacuating the air between the emulsion layer and the etchant resist layer. In the prior art processes the emulsion pattern strip lines usually have had sufficient breaks or irregularities so that gaps or breaks in the emulsion pattern strip line permit evacuation of the air from within the region defined by the strip line. With the present invention we have discovered that we do not need to rely on irregularities in the emulsion master pattern of the strip line. We preserve the accuracy of etchant resist pattern by application of a set of spacers on top of the soft emulsion base to provide a path for air to escape past the strip line. The use of spacers projecting upward from the emulsion base without any detrimental degradation of the image developed therefrom is not fully understood in view of the problems encountered with foreign particles that occasional come into contact with the tops of the opaque master pattern image on the emulsion. The foreign particles on top of the emulsion projections usually result in fracture of the emulsion opaque image when a vacuum is drawn between the emulsion layer and the etchant resist layer thus rending the photo printing plate unsuitable for use in laying out an accurate etchant resist pattern on the etchant resist on the metal web.